If this is what I'm thinking of, arnwyn is correct. There was no product made to support the Return to the Tomb of Horrors however there was a promotional pamphlet. This pamphlet most likely went to retailers and might have been passed out at Gencon or whatever convention preceeded the release of RttToH.
Now this was not just a sales advertisement pamphlet if I recall. It was a story or journal or map from a thief who briefly stole something from the Necromancers City or maybe Deseyto's stronghold. Actually, I think the story connecting it to the RttToH came after the pamphlet was passed around because people were so intrigued with the idea they wanted to know the link between it and the Tomb. Of course they may have had the story idea before releasing it, I simply don't remember all the details anymore.